The present invention relates to optical fiber communication equipment, an optical signal receiver and an optical signal receiving module.
An optical signal receiver may consist, for instance as shown in FIG. 1, an optical signal receiving module 100, an automatic gain control (AGC) amplifier circuit 40 for carrying out equalizing amplification, a decision circuit 41 for converting the amplified analog signals into digital signals and supplying them, and a re-timing circuit 42 for performing clock regeneration. The optical signal receiving module 100 consists of optical components such as a photo detector, a pre-amplifier circuit, a bias circuit for photo detector, optical fibers and a lens. In FIG. 1, illustration of optical components including optical fibers is omitted. Regarding the AGC amplifier circuit 40, it may be included in the optical signal receiving module 100 or, if the gain of a pre-amplifier circuit 10 is sufficient, the AGC amplifier 40 will be unnecessary. Furthermore, where the dynamic range required for reception sensitivity is narrow, a simple amplifier circuit having no AGC function may be used.
The principal performance of the optical signal receiver is dominated by the input equivalent noise current and frequency characteristic of the optical signal receiving module 100. Both these input equivalent noise current and frequency characteristic are characteristics in the high frequency region. In an optical signal receiving module for high speed optical transmission, the influence of parasitic elements attributable to the mounted state of a photo detector 1, the pre-amplifier circuit 10 and a photo detector bias circuit 30′ is particularly large. Parasitic elements in the optical signal receiving module 100 include parasitic inductances and parasitic capacitances related to the connection or mounting of the photo detector 1, the pre-amplifier circuit 10 and the photo detector bias circuit 30′. Usually, in order to reduce parasitic elements related to connection such as inductances, it is desirable to connect the photo detector 1 and the pre-amplifier circuit 10 in mutually as close positions as practicable. In other words, it is necessary to keep the parasitic inductances attributable to the connection of the photo detector 1 and the pre-amplifier circuit 10, capacitances including the junction capacitance of the photo detector 1 and the input terminal capacitance of the pre-amplifier circuit, and the resonance frequency of LC resonance based on the parasitic capacitance sufficiently high relative to the transmission frequency band even where the transmission band is expanded with an increase in transmission speed. In order to reduce the parasitic inductance attributable to the connection of the photo detector 1 and the pre-amplifier circuit 10, the bonding lead length between the photo detector 1 and the pre-amplifier circuit 10 should be shortened. For instance, JP-A-58881/2000 discloses a technique for reducing parasitic inductances by mounting the photo detector and the pre-amplifier circuit on the same chip carrier.
Generally, photo detectors and pre-amplifier circuits are devices susceptible to fluctuations in characteristics. For this reason, the photo detector part and the pre-amplifier circuit part are separately screened. Then, only the components that have passed the screening are assembled, and the yield of acceptable optical signal receiving modules can be significantly enhanced thereby. Also, if data on the temperature-dependence of a photo detector are required, the data will have to be obtained for the photo detector alone. For instance, where an avalanche photo diode (APD) is used as a photo detector, because the temperature-dependence of the breakdown voltage or the dark current differs from one APD to another, data acquisition for the photo detector alone is indispensable. In the structure described in JP-A-58881/2000 cited above, it is difficult to evaluate the photo detector as an isolated chip. On the other hand, a configuration in which a photo detector is mounted on the chip carrier side and a pre-amplifier circuit on the printed circuit board is disclosed in, for instance, the U.S. Pat. No. 5,200,612. In this configuration, no other active element than the photo detector is mounted on the chip carrier side, and therefore, the photo detector part can be screened. However, this would entail a greater length of the lead for connecting the photo detector and the pre-amplifier circuit with a consequence of an increase in the parasitic inductances noted above.
Also, for the optical signal receiving module, the following requirements are specified to achieve a prescribed level of reception sensitivity:
(1) The return loss of the received optical signals should be kept at or below a certain level (for instance, not more than −27 dB according to the ITU-T standard) to maximize the coupling efficiency of the photo detector and the optical fiber;
(2) The density of the input equivalent noise current of the pre-amplifier circuit should be minimized:
(3) In connection with (2) above, the junction capacitance of the photo detector should be minimized; and
(4) The frequency response characteristics should be optimized relative to circuits downstream.
Regarding the frequency response characteristics mentioned in (4) above, S parameters S21 and S22 after photo electric conversion should satisfy respectively prescribed requirements. While depending on the characteristics of circuits downstream, usual S parameter requirements for operation at 10 Gbits/s of NRZ signal as transmission code are, for the S21 characteristic, (A) 3 dB band≧7 GHz and (B) intra-band deviation ≦±1 dB, and for the S22 characteristic, (C) S22≦−7 dB (at 10 GHz). These S21 characteristic and S22 characteristic suppose the use of port 1 as the optical signal input terminal and port 2 as the electrical signal output terminal. In a configuration in which the photo detector is mounted on the chip carrier and the pre-amplifier circuit is mounted on the printed circuit board, as described above, the characteristics of (A) and (B) stated above are significantly affected by the relationship between the parasitic inductance arising as a result of the connection of the two elements and other parasitic capacitances. The technique disclosed in JP-A-2000-58881 is intended to improve these characteristics by mounting the photo detector and the amplifier circuit on the chip carrier and thereby shortening the lead for connecting them and accordingly reducing the resultant parasitic inductances. However, there is a limit to the reduction of parasitic inductances, which can never be reduced to zero. The cited patent application does not state what is to be done when peaking or dipping of S21 has arisen in a frequency band where the characteristic is affected by LC resonance occurring between this inductance and capacitance.
FIG. 3 shows an equivalent circuit around the chip carrier of an optical signal receiving module. In this diagram, reference numeral 1 denotes a photo detector; 10, a pre-amplifier circuit; 11, a bypass capacitor; 25, the input capacitance of the pre-amplifier circuit; 30, a bias power supply for photo detector; 31, a parasitic inductance due to the bonding wire between the bypass capacitor 11 and the electrode pattern of the bias power supply; 32, parasitic inductance due to the bonding wire between the bypass capacitor 11 and a pattern on the chip carrier; 33, the junction capacitance of the photo detector; and 34, a parasitic inductance due to the bonding wire between the pre-amplifier circuit 10 and the pattern on the chip carrier. A reduction in parasitic elements accountable for these parasitic inductances would be effective for enhancing the frequency characteristic. It is conceivable, for instance, to mount both the photo detector 1 and the pre-amplifier circuit 10 on the chip carrier to shorten the bond wiring length and thereby to reduce the parasitic inductances. However, without evaluating the photo detector 1 part in a separate state from re-amplifier circuit 10, no initial trouble with the photo detector 1 can be detected by screening. For a configuration in which both the photo detector 1 part and the pre-amplifier circuit 10 are mounted on the chip carrier, any module defect would be found in a subsequent process, and this would bring down the productivity of modules, to avoid which the photo detector should be mounted on the chip carrier and the pre-amplifier circuit, on the printed circuit board.
Problems to be solved by the present invention, in view of the state of the related art described above, include: (1) how to free even an optical signal receiving module, wherein the photo detector and the pre-amplifier circuit are not mounted on the chip carrier and accordingly parasitic elements cannot be reduced, from peaking and dipping and provide it with frequency characteristics in the necessary and sufficient band, and (2) how to make it possible to suppress peaking or dipping in accordance with the results of advance evaluation of a photo detector and a pre-amplifier circuit whose characteristics differ from wafer to wafer or from one process lot to another, limit the band of frequency characteristics to prevent it from becoming too wide, and thereby to prevent circuit noise components from increasing and the reception sensitivity from deteriorating.